simply refer to other well-known specifications. For example, the lower layers all support the
IEEE standards for Ethernet and Token Ring, the ANSI standard for FDDI, the ITU standard
for ISDN, and the Frame Relay protocols specified by the Frame Relay Forum, ANSI, and the
ITU. The protocol stacks can accommodate other evolving Layer 1 and Layer 2 specifications
more easily by referring to emerging international standards rather than trying to evolve these
standards themselves.
chapter are error recovery and flow control. Routers discard packets for many reasons,
including bit errors, congestion that has caused a lack of buffer space, and instances in which
no correct routes are known. The transport layer can provide for retransmission (error recovery)
and can help avoid congestion (flow control).
oriented, so CCNAs deal with the concepts of connectionless and connection-oriented
protocols on a regular basis. This next section compares the two and provides some explanation
for the functions of each. Error recovery and flow control are covered in the section "How Error
Recovery Is Accomplished."
connotations inside the world of networking protocols. However, the typical connotation can be
a bit misleading. For instance, most people correlate connection-oriented protocols with
reliable or error-recovering protocols because the two features are often implemented by a
single protocol. However, connection-oriented protocols do not have to provide error recovery,
and error-recovering protocols do not have to be connection-oriented.
before data transfer begins or has a required pre-established correlation between two
endpoints.
that does not require a pre-established correlation between two endpoints.
oriented because a set of three messages must be completed before data is exchanged. Likewise,
SPX is connection-oriented. Frame Relay, when using PVCs, does not require any messages be
sent ahead of time, but it does require predefinition in the Frame Relay switches, establishing a